Laser isotope separation (LIS) enrichment processes are used for producing isotope enriched uranium metal for use as fuel in nuclear reactors. The LIS process requires uranium alloys as feed to the separation process rather than uranium hexafluoride (UF.sub.6). Depending on the method used to prepare the uranium feed for the LIS process, the cost of producing the feed can be a significant contributor to the overall costs of the uranium enrichment process.
Currently, uranium feed for LIS processes is produced by the metallothermic reduction of uranium tetrafluoride (UF.sub.4) by either the Ames Process or the Elliott Process. The Ames process is a two step batch process for converting UF.sub.6 to uranium metal. First, in a continuous process, UF.sub.6 is reduced with hydrogen to form UF.sub.4. Anhydrous hydrogen fluoride (HF), a valuable by-product, is also produced. The second step involves the batch process conversion of UF.sub.4 to uranium metal. This process involves blending UF.sub.4 with magnesium metal chips in a graphite or magnesium fluoride lined reaction vessel. The contents of the vessel are then slowly heated for 4 hours to about 540.degree. C. at which point the following reaction occurs over the course of about 2 minutes: EQU UF.sub.4 +2 Mg.fwdarw.U+2MgF.sub.2.
This reaction is highly exothermic, providing sufficient heat to liquify the products. The liquid uranium that is generated forms droplets that fall through the liquid MgF.sub.2 slag to form a mass of uranium metal at the bottom of the vessel.
In order to recover the generated uranium metal, the vessel must first be allowed to cool for several hours. Afterwards, the uranium is separated from the MgF.sub.2 by impact methods. Separation of the uranium from the MgF.sub.2 slag is incomplete, resulting in a significant loss of the valuable uranium metal product. In addition, the uranium content in the resulting MgF.sub.2 slag is sufficiently high so as to require the slag to be disposed as low-level nuclear waste. The MgF.sub.2 slag waste generated is many times as voluminous as the uranium metal generated and weighs roughly one-half of the weight of the uranium that is generated. Thus, the Ames method has several significant shortcomings.
Several of the shortcomings of the Ames process are overcome by the Elliott process. The Elliott process is a multistep process involving the reduction of UF.sub.4 by magnesium metal. The reduction reaction is conducted in a rotary furnace at a temperature of about 1000.degree. C. wherein the generated solid uranium metal particles are dispersed in solid MgF.sub.2. The uranium metal is separated from the MgF.sub.2 salt in the second stage of the process wherein the mixed uranium-MgF.sub.2 product is mixed with CaCl.sub.2 in a reactor at 1150.degree. C. to yield uranium metal and MgF.sub.2 --CaCl.sub.2. The process can be operated continuously by separately removing the uranium metal product and mixed salt by-product.
While more efficient than the Ames process, the Elliott Process has the disadvantages that it requires higher volumes of mixed salt and an additional reheat step to melt the uranium and separate the uranium from the residual salt.
Uranium metal has also been produced electrochemically. Glassner et al. reprocessed spent uranium fuel using a molten KCl--LiCl--UF.sub.4 electrolyte bath at 425.degree. C. wherein the solid uranium metal product is deposited on a Mo electrode. Glassner et al., Chemical Engineering Division Summary Reports, ANL-4872, p. 147 (1952). Martin et al. used a KCl--UCl.sub.3 electrolyte bath at 900.degree. C. to cause solid purified uranium to deposit on a Mo electrode. F. S. Martin, G. L. Miles, "Process Chemistry"1, p. 329 (1956). Niedrach et al. used a BaCl.sub.2 --UF.sub.4 electrolyte bath with a Ni and Mn electrodes at 975.degree.-1075.degree. C. to prepare purified uranium metal. L. W. Niedrach et al. In. Eng. Chem. 48, 977 (1956); L. W. Niedrach et al., KAPL-1692 (1957); L. W. Niedrach et al., "Process Chemistry" 2, p. 396 (1958). K. Cho et al. prepared solid uranium metal at 600.degree.-750.degree. C. using a UCl.sub.3 --KCl--NaCl electrolyte bath, an U--Nb alloy electrode and a Mo electrode. K. Cho et al., Denki Kagaku 37 (11) 791-795 (1969). Solid uranium metal was formed on the Mo electrode surface by this process with an electrode current efficiency of 50-90%. Piper (Production of Uranium Metal from Uranium Oxide by Fused Salt Electrolysis, Electrochemical Technology, March-April 1967, pp. 147-151) describes electrolytic processes of the production for molten uranium metal using a BaF.sub.2 --LiF--UF.sub.4 electrolyte.
There have also been some studies in which low melting zinc has been used as a molten electrode to recover the uranium as an intermetallic compound. O. F. Brand et al., A Conf., 15, p. 1780 (1958); Takasi Mukaibo et al., Nippon Gonshiryoku Gakkaishi 7, No. 8, 410 (1965), however, this requires further removal of zinc by evaporation.
Thus, there is a need for a way to produce relatively large quantities of uranium metal in a way that is cost effective yet produces fewer environmentally undesirable side-products. There is a further need for a system which is relatively compact, less costly from a capital standpoint and which can produce a continuous product stream.